Clue to why some die during sleep
They say a cumulative loss of cells in the area of the brain that controls breathing is to blame - triggering a condition called central sleep apnoea.
However, they believe many such deaths in elderly people are misdiagnosed as heart failure.
The study, by the University of California, Los Angeles, is published in Nature Neuroscience.
The researchers had previously pinpointed a region of the brainstem they dubbed the preBötzinger complex (preBötC) as the command post for generating breathing in mammals.
They had also identified a small group of cells within this area as being responsible for issuing the commands.
In the latest study, they injected rats with a compound to kill more than half of these cells - and then monitored the animals' breathing patterns.
When the animals entered the rapid eye movement phase of sleep - when dreaming occurs - they stopped breathing completely, and were jolted into consciousness in order to start again.
Over time, the breathing lapses increased in severity, spreading to other phases of sleep, and eventually occurring when the animals were awake as well.
Rats possess 600 of the specialised cells. The researchers believe humans have a few thousand, which are slowly lost over a lifetime.
Lead researcher Professor Jack Feldman said: "We speculate that our brains can compensate for up to a 60% loss of preBötC cells, but the cumulative deficit of these brain cells eventually disrupts our breathing during sleep.
"There's no biological reason for the body to maintain these cells beyond the average lifespan, and so they do not replenish as we age.